1. The Field of the Invention
This invention relates generally to x-ray imaging apparatus. More specifically, the invention relates to automatic brightness control in a closed loop imaging system which uses an automatic brightness control sampling window.
2. The State of the Art
X-ray imaging systems are well known medical diagnostic and interventional tools. X-rays are generated when a high voltage generator supplies electric power to an x-ray tube. One circuit prepares the tube for x-ray exposure by heating the tube filament. A second circuit generates a high voltage potential that accelerates x-rays from the filament (or cathode) to the anode within the x-ray tube.
The filament in the x-ray tube is a coiled tungsten wire that, when heated by current flow, emits electrons. This is a low voltage circuit. Relatively little power is needed to heat the filament, and small variations in filament current result in large variations in x-ray tube current.
Electrons emitted from the filament are focused onto a spot in the tungsten target (anode). X-ray photons are produced when the electrons interact, by sudden deceleration, within the tungsten anode. The target surface is angled to reflect the x-rays in the direction of the x-ray tube output window.
In order to understand the relationship between the high voltage generator control and its effects on x-ray beam penetration, and subsequently, diagnostic image quality, it is important to remember that the intensity of an x-ray beam varies with the electrical potential in kilovolts (kVp) and the tube current (mA) applied to the tube. The quality and intensity of the x-ray photons generated depends almost entirely on the kVp.
Because exposure to radiation is harmful, all practical methods are employed to reduce x-ray exposure to patients. One method is to collimate the x-ray beam with materials that will partially or completely absorb x-rays. Proper beam collimation also assists the video imaging system by prohibiting non-attenuated (unimpeded) x-ray photons from reaching an image intensifier.
Varying clinical procedures have unique beam collimation requirements. The most common systems employ a combination of fixed, adjustable leaf, and adjustable iris collimation. Systems can also use a combination iris and leaf collimator with independent motorized controls and position feedback.
During the examination of a patient, an image is produced when x-ray photons pass through the patient and are then converted to light photons through the use of an image intensifier tube or some other x-ray conversion device. The x-ray photons pass through tissue and materials of varying mass and composition before striking the input surface of the image intensifier. X-rays will either penetrate or be absorbed by whatever lies in the path of the X-ray beam. As x-rays strike the input screen in the image intensifier, x-ray photons are converted into light photons. Within the photocathode the light photons are converted to electrons and focused by an electrostatic lens onto an output phosphor. The output phosphor once again converts the electrons to light photons where the image is visible at the output window of the image intensifier tube.
At this stage, calculating the actual sizes or dimensions of objects in the image requires knowing: 1) the image intensifier size, 2) the image intensifier electrostatic lens magnification, and 3) the magnification of the object due to its distance from the surface of the image intensifying tube.
A video camera captures the image as it is displayed at the output of the image intensifier. The automatic brightness system (ABS) control application dynamically determines the camera gain, kVp and mA, based on the image brightness statistics. Peak brightness and average brightness of the area within the ABS sampling window are used as the conventional factors in setting the ABS control parameters (kVp, mA and camera gain). It should also be mentioned that the image intensifier might also be some other type of image receptor such as a flat panel x-ray image receptor or some other scanned image x-ray receptor.
As a patient is re-positioned beneath the x-ray beam during an examination, the brightness of the video image changes because of variations in the attenuation of the x-ray beam as it passes through different thicknesses and densities of body tissue and bone. In order to compensate for these variations in image brightness, various automatic brightness compensation systems have been devised.
For example, in U.S. Pat. No. 4,573,183, the automatic brightness control derives the average brightness of the image from the video signal. That average brightness information is used to produce a video gain signal for controlling the camera and to generate a brightness feedback signal that controls an x-ray tube power supply. In response to the brightness feedback signal, the x-ray tube power supply generates a bias voltage and filament current for the x-ray tube to thereby regulate brightness of the x-ray image formed by the image intensifier tube. By varying the gain or the aperture of the video camera, the resulting image brightness on the monitor also is controlled. Accordingly, the feedback provided to the automatic brightness control modifies the x-ray emission and camera gain which affects the image brightness. The feedback control can be calibrated to generate consistent image display brightness regardless of changes in patient x-ray absorption. The standard automatic brightness control derives the video gain and brightness feedback signals from a common average brightness value, a peak brightness, or a combination of the values for the image. However, a common brightness feedback signal results in less than ideal control of these system parameters which affect the image generated on the monitor.
Another example of the state of the art is taught in U.S. Pat. No. 4,703,496. This patent apparently teaches that luminances of picture elements in each video image field were averaged to generate a signal having a voltage proportional to the average image brightness.
The average brightness value is used as a feedback signal to control the excitation of the x-ray tube and the video gain to thereby maintain the video image brightness substantially constant at an optimum level. However, the system is relatively complex in that it utilizes three separate loops for regulating tube current, bias voltage and video gain.
The systems described are complex and are difficult to maintain. Accordingly, it would be an advantage over the state of the art to provide an automatic brightness control system which is simpler to operate, and is able to compensate for the effects of improper patient positioning, variations in patient size, and rapid changes from torso to extremity imaging.